BANGKOK (AFP) - A key suspect in last month's deadly Bangkok blast paid a US$600 (S$850) bribe to illegally enter Thailand, police said on Thursday (Sept 10), highlighting widespread corruption at the kingdom's borders.

Adem Karadag, one of two foreigners arrested in connection with the Aug 17 attack, was detained at a flat in eastern Bangkok late last month.

Police say he was found in possession of bomb-making paraphernalia and dozens of fake Turkish passports.

Karadag's role has not been explained by police, but say they now know more about how he came to be in Thailand.

"The first man (Karadag) said he travelled through Vietnam to a neighbouring country where he then paid for transportation," police chief Somyot Poompanmoung told reporters, without specifying which neighbouring country.

"At the Thai border he paid US$600 (to cross into Thailand)," he added, without revealing the identity of the official who took the bribe.

A second man, Yusufu Mieraili, was detained two days later on the border with Cambodia.

He was allegedly found in possession of a Chinese passport which police believe is genuine.

Police say Mieraili has confessed to delivering the backpack bomb to another man who left it at the Erawan Shrine minutes before the explosion.

Thailand is a notorious sanctuary for on-the-run foreigners and visa over-stayers, with officials often willing to take a bribe to turn a blind eye to illegality.

In recent days Somyot, himself a former deputy commander with immigration police, has railed against the ease of buying off border officials.

"I cannot ignore this problem because I feel ashamed," he told reporters on Wednesday as he called on the junta to help him clamp down on corrupt border officials.

Although Somyot did not confirm which country Karadag entered from, it is likely to be Laos or Cambodia, both of which sit between Thailand and Vietnam.

Since the bombing at least six police officers have been removed from their posts bordering Cambodia where Mieraili was apprehended after they reportedly took bribes to let people pass.

Somyot raised eyebrows shortly after Karadag's arrest when he announced that he was awarding his own officers a reward of some US$84,000 for making their first arrest in the case. At the time Karadag had not even been charged.

Mystery still surrounds the motive of the group accused of being responsible for the August 17 bombing that left 20 people dead in the heart of Bangkok and rocked Thailand's key tourist industry.

Analysts have suggested a link with Muslim Uighur militants or their supporters, possibly enraged by Thailand's deportation of scores of the minority to their Chinese homeland earlier this summer.

In recent days Thai media have focused on a suspected mastermind identified as "Izan" or "Ishan" who reportedly orchestrated the bombing and fled the country before the attack.

Police on Thursday confirmed the man was a suspect but played down his status.

"Do not conclude Ishan was a big fish," Somyot said. "You might be surprised."